heater is blowing cold air when on hot have replaced radiator and flushed? on 2000 Ford Expedition

what could cause this and do you have a D.I.Y. solution to this issue.is this a common problem

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2 answers
Check the heater to ensure the trash you just flushed didn't go into the core and stop it up as well. Next would be to check/test the temp/blend door actuator.
also ck coolant level